Hi Faizan,
You might be missing a couple of setting for TACACS to work. First, you need to tell the device what TACACS device to use with this setting: tacacs-server host x.x.x.x. Next, if you are using a shared secret with your TACACS configuration, you will need this setting: tacacs-server key 7 ABCDEFGHIJKLMNOPQRSTUVWXYZ. Last, To send only a username to a specified server when a direct request is issued, use the tacacs-server directed-request command in global configuration mode.